Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 35
न त्वं विस्मृतशस्त्रास्त्रान् विरथान् भयसंवृतान् ।हंस्यन्यासक्तविमुखान् भग्नचापानयुध्यत: ॥ ३५ ॥
na tvaṁ vismṛta-śastrāstrānvirathān bhaya-saṁvṛtānhaṁsy anyāsakta-vimukhānbhagna-cāpān ayudhyataḥ
When the demigods are bereft of their chariots, when they forget how to use weapons, when they are fearful or attached to something other than fighting, or when their bows are broken and they have thus lost the ability to fight, Your Majesty does not kill them.
There are principles that govern even fighting. If an enemy has no chariot, is unmindful of the fighting art because of fear, or is unwilling to fight, he is not to be killed. Kaṁsa’s ministers reminded Kaṁsa that despite his power, he was cognizant of the principles of fighting, and therefore he had excused the demigods because of their incapability. “But the present emergency,” the ministers said, “is not intended for such mercy or military etiquette. Now you should prepare to fight under any circumstances.” Thus they advised Kaṁsa to give up the traditional etiquette in fighting and chastise the enemy at any cost.